Chofetz Chaim Daily 19 Elul 9:5,6 When you are allowed to tell R about a potentially bad partner, thats only if theyre not yet partners, if theyre already partners it depends; if R is the type to immediately break the partnership, then you cant tell him. If he will just protect himself from losses then its ok.
Chofetz Chaim Daily 18 Elul 9:3 If you know that R is plotting to hurt S, you must tell S provided fulfillment of aforementioned conditions. 4) you must be sure that by telling S, hell stay away from R, but if hes type to make preemptive strike on R and make a bigger fight, then you cant tell him.
Chofetz Chaim Daily 17 Elul 9:1 If you know bad things about the ploni that R wants to enter partnership with, you must tell R, provided that you fulfill these conditions A. its definitely destructive to partner B. must not exaggerate C. must intend ltoeles (be sure that R will actually take your advice) D. seek alternative methods first E.